This invention generally relates to automatic frequency control (AFC) loops which are utilized to automatically track or adjust the frequency of a received signal. This invention more specifically addresses the AFC loop filter and the effect of the closed loop frequency response of the AFC loop on the information recovered from a received signal by using an AFC circuit.
Various types of AFC circuits are well known. A feedback network responsive to a detected frequency error of a desired signal is utilized by an AFC circuit to change the frequency of a generated signal to minimize the detected frequency error.
This invention is particularly, but not exclusively, suited for use in a wireless communications receiver with an AFC circuit which minimizes the frequency error of a received FM signal. In such an application, a low pass filter has been utilized in the feedback loop of an AFC circuit to filter the recovered audio signal to provide an error signal. For example, see U.S. Pat. No. 3,764,917 issued to Dong Woo Rhee, and an article entitled "A Digital Mobile Radio for 5-6 Kilohertz Channels" by Scott Carney and Don Linder published June, 1982, in Vol. 3 of the IEEE International Conference on Communications.
A disadvantage in utilizing a conventional one pole low pass filter in an AFC loop is that the AFC closed loop frequency response causes attenuation of low frequencies in the recovered information. This results in degradation in the recovery of digital signals.